1. Field of the Invention
The invention relates generally to the liquid and gas ultrasonic flow meters. More particularly, the invention relates to a robust electrical connection to the piezoelectric crystal of a transducer employed in an ultrasonic flow meter.
2. Background of the Technology
After hydrocarbons have been removed from the ground, the fluid stream (either in a liquid phase or a gaseous phase) is transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or during “custody transfer.” Even where custody transfer is not taking place, however, measurement accuracy is desirable, and in these situations ultrasonic flow meters may be used.
An ultrasonic flow meter typically includes two or more transducer assemblies, each secured inside of a port in the body of the flow meter. The body of the flow meter may also be referred to as a spool piece. To contain the transported fluid within the flow meter, a connector is secured over the exterior end of each transducer port in the spool piece. Thus, the spool piece and end connectors create a pressure boundary and housing that contains the fluid flowing through the meter.
To measure fluid flow through the meter, a pair of transducer assemblies is positioned along the inner surface of the spool piece, such that each transducer assembly faces the other on opposite sides of the fluid flow through the bore of the spool piece. Each transducer assembly includes a piezoelectric element. When an alternating current is applied to the piezoelectric element of the first transducer assembly of the pair, the piezoelectric element responds by radiating an ultrasonic wave through the fluid flowing through the flow meter. When the wave is incident upon the piezoelectric element of the second transducer assembly of the pair, the second transducer assembly responds by generating an electric signal. Some time later, an alternating current is applied to the piezoelectric element of the second transducer assembly, and the second piezoelectric element responds by radiating an ultrasonic wave through the fluid in the flow meter to the first transducer assembly. When the wave is incident upon the piezoelectric element of the first transducer assembly, that transducer assembly responds by generating an electric signal. In this way, the transducer assemblies transmit and receive signals back-and-forth across the fluid stream.
Each transducer assembly is connected to a cable that extends through the end connector to the exterior of the spool piece and a remote location, such as an electronics base enclosure typically mounted to the outside of the spool piece. The cable carries the electric signal created by the piezoelectric element of the particular transducer assembly to an acquisition board positioned within the electronics base enclosure or electronic package, where the signal may be processed and subsequently used to determine the fluid flow rate through the meter.
Electrical connections to the piezoelectric element allow for the communication of electrical signals to and from the piezoelectric crystal. During operations of an ultrasonic flow meter, temperature fluctuations are a common source of stress on the electrical connections to the piezoelectric element. For example, liquid ultrasonic flow meters can have temperature ratings of −50 to +150° C. or greater and their transducers need to be able to remain functional over the full temperature range. Additionally, ultrasonic flow meters that measure cryogenic liquids such as liquid natural gas (LNG) may experience temperature swings between ambient temperature (approximately 20° C.) or higher and the temperature of the LNG (approximately −161° C.) or lower. Although efforts are made to keep the rate of temperature change low (i.e., keep the change in temperature per unit time low), rapid temperature changes (i.e., thermal shock) can occur. Temperature changes can exert stress on the electrode connections through differences in the coefficients of thermal expansion (CTEs) of the various materials making up the transducer.
The piezoelectric crystal contains two electrodes—a negative electrode and a positive electrode. Typically, an electrical connection to each electrode is accomplished by soldering a wire directly to each electrode. The piezoelectric crystal is held in place in the crystal holder of the transducer assembly using epoxy that also acts as a back matching layer to improve acoustical performance by increasing the bandwidth of the transducer.
Epoxy is often used to encapsulate all or a portion of the piezoelectric crystal and associated wires. In addition to improving acoustical performance, the epoxy also helps to protect the piezoelectric crystal, wires, and electrical connection from corrosion and mechanical shock. The epoxy encapsulation does have a downside, however, in that the CTE of the epoxy typically differs substantially from that of the piezoelectric crystal. Epoxies typically have CTEs of 15 to 100 ppm/° C., while a piezoelectric material such as lead zirconate titanate (PZT) typically has a CTE of approximately 3.6 ppm/° C. For a given temperature change, the epoxy dimensions change more than the piezoelectric crystal dimensions. The net effect is that the dimensional changes of the epoxy tend to exert a force at the point where the wire is attached to the electrode and can cause a portion of the electrode material to break apart from of the piezoelectric material.
The fact that the electrical connection to the electrode tends to be kept as small as possible further worsen the stress problem since it concentrates the stress along a relatively small surface area the electrode. For example, if the electrical connection is a solder joint, it is typically kept as small as possible to minimize heating of the electrode and piezoelectric crystal since too much heat from soldering iron can cause damage to the electrode or piezoelectric material. If the electrical connection is accomplished with conductive epoxy, the electrical connection can be made a little larger by spreading out the individual strands within the wire to increase the area of contact with the electrode but this does increase the difficulty in making the electrical connection.
An alternative method to making the electrical connection to the electrode is to use a spring or spring-loaded mechanism that biases the electrical connection into engagement with the electrode such that the connection can freely slide over the electrode of the piezoelectric crystal. However, this type of electrical connection can degrade over time due to corrosion of the electrode material and/or connection mechanism causing the electrical resistance of the connection to increase over time.
Accordingly, there remains a need in the art for a more robust electrical connection to the electrodes of a piezoelectric element of a transducer assembly. The electrical connection would be particularly well received if it offered the potential to withstand thermal and mechanical stresses experienced during operation over a relatively broad range of temperatures and applications.